1. Field of the Invention
This invention relates to damping shimmy of a rotary member relative to an associated member and is more particularly concerned with controlling and avoiding shimmy of casters of tye type which may be used in supporting wheelchairs, pushcarts, and the like.
By way of example, conveyances such as wheelchairs, pushcarts, and the like, may be run over hard surfaces, e.g. hospital corridors, where shimmying of the caster wheels may generate undesirable noise. Casters for such purposes are generally fairly small. The yokes for such casters may be on the order of 11/2" outside spread dimension, and the swivel axles may be on the order of about 1/2" diameter of less and on the order of three times as long as the diameter. Therefore, equipping such casters with shimmy attenuating means involves a substantial problem in miniaturization.
Large size shimmy dampers have heretofore been employed for aircraft landing wheel assemblies, as represented in U.S. Pat. No. 2,661,917 employing the principle of torque resistance of shear films of a suitable viscous fluid operating between opposed parallel radially extending working surfaces relatively rotatably movable in their respective planes and wherein certain of the working surfaces are attached to a landing wheel strut and other of the surfaces to a wheel yoke. In such landing wheel assemblies there is ample room for the diameter to which the damper may extend and the lateral projection of the damper was found useful in effecting scissors connection between the strut and the yoke.
Also useful in the airplane industry, flutter dampers such as represented in U.S. Pat. No. 3,228,494 have employed the shear film principle, utilizing axially extending complementary shear film spaced working surfaces on relatively rotatable members one of which may be attached to a relatively fixed part of the aircraft and the other attached to a relatively movable control surface in the wing or tail assembly for eliminating flutter. In this instance, as in the foregoing prior art example, relatively larger damper units are involved.